SSL8516T All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 1 — 5 May 2014 13 of 33
NXP Semiconductors
SSL8516T
Greenchip PFC and flyback controller
Valley switching is active in all operating modes.
To prevent high frequency operation at lower loads, the quasi-resonant operation switches
to discontinuous mode operation with valley skipping. When the frequency limit is
reached, the quasi-resonant operation changes to DCM with valley skipping. The
frequency limit reduces the MOSFET switch-on losses and conducted EMI.
At medium and low power levels, the controller enters Frequency Reduction (FR) mode. A
Voltage Controlled Oscillator (VCO) controls the frequency. During frequency reduction
mode, the primary peak current is kept at an adjustable minimal level to maintain a high
efficiency. Valley switching is also active in this mode.
In frequency reduction mode, the PFC controller switches off when the flyback switching
frequency has dropped to below 53 kHz and V
PFCTIMER
has reached 3 V. The flyback
maximum frequency changes linearly with the control V
FBCTRL
(see Figure 8). Hysteresis
is added to ensure a stable PFC switch-on and switch-off. In no-load operation, the
switching frequency is reduced to (almost) zero.
7.3.2 Valley switching (HV pin)
A new cycle starts when the external MOSFET is switched on. V
FBSENSE
and V
FBCTRL
determine the on-time. The MOSFET is then switched off and the secondary stroke starts
(see Figure 9
). After the secondary stroke, the drain voltage shows an oscillation with a
frequency of approximately:
(3)
where L
p
is the primary self-inductance of the flyback transformer and C
d
is the
capacitance on the drain node.
When the secondary stroke ends and the internal oscillator voltage is high again, the
circuit waits for the lowest drain voltage before starting a new primary stroke.
Fig 8. Flyback frequency control
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